In the prior art "choke screws" have been used for choking a current of gas, for example compressed air. However such screws have a number of different shortcomings, one being for example that they have a very unlinear characteristic, that is to say when the cross-section is small a great change in the choking effect is produced on adjustment while at the other end of the range with a high cross-section only a small change is produced on adjustment. Furthermore, because the chokeway is long and narrow, there is a danger of its becoming stopped up, for example when used for dirty gases. There is the further shortcoming that a choke screw has a high frictional effect.